In general, as shown in FIG. 1, in a sewing machine, a rotary hook 100 for supplying a lower thread is installed. Such a rotary hook 100 is mounted in a driving shaft 10 rotatably connected to a power apparatus (power transfer gear) of the sewing machine and includes a hook body 20 called an outer rotary hook, a bobbin case (hereinafter, referred to as a “BC”) received in the hook body 20, and a bobbin 40 in which a lower thread is wound.
The BC includes a BC base 30 housed in the hook body 20 and called an inner rotary hook and a BC body 50 for receiving a bobbin at the outside of the bobbin 40.
At the bottom of the BC base 30, a stud 31 is provided, and at the stud 31, the bobbin 40 is rotatably installed.
The BC formed with the BC base 30 and the BC body is integrally formed, and a protrusion 75 of a hook retainer 70 separately installed in a sewing machine body is inserted into a groove 33 to formed in the BC base 30 to prevent the BC from moving. Further, a needle through-hole 34 that is adjacent to the groove 33 and that penetrates a needle when the needle moves downward is formed.
In the bobbin 40, a winding shaft 41 in which the lower thread is wound, and flanges 43 and 45 formed at both ends of the winding shaft 41 are formed.
When the hook body 20 is driven by such a structure, the hook retainer 70 prevents the BC base 30 from rotating along the hook body due to a rotation of the hook body 20 and thus the BC base 30 maintains a fixed state, and an upper thread 3 that is hooked to a hook 21 of the hook body 20 and that revolves over the BC base 30 and the bobbin 40 and that moves upward by hooking a lower thread 4 is guided through the groove 33 (precisely, between a side wall of the groove and the protrusion 75 of the hook retainer within the groove) of the BC base (see FIG. 2).
Therefore, the groove 33 functions as a key groove that inserts the protrusion 75 of the hook retainer, and a width 33a thereof is added to a width 75a of the protrusion not to have a trouble when the upper thread 3 escapes and thus the groove 33 is formed in a size for securing an upper thread passage gap.
In such a conventional rotary hook 100, when the lower thread wound in the bobbin 40 is consumed, operation of the sewing machine is stopped and the BC body 50 is opened, and the bobbin 40 should be replaced, and in order to replace one bobbin 40, about 1 minute is generally consumed.
When it is assumed that a length of the lower thread wound in the bobbin 40 is 40 meter, a length of a stitch is 1 millimeter, and the reciprocating rotation motion number of a needle bar is 4000 RPM, if 10 minutes have elapsed, entire lower thread is consumed and thus the sewing machine should be stopped every 10 minutes and the bobbin 40 should be replaced, and in a large-sized sewing machine in which a plurality of (e.g., 50 to 100) needle bars and rotary hooks 100 are disposed in a line in a transverse direction, if a wound lower thread of any one of the bobbins 40 is consumed, operation of the sewing machine should be stopped and the entire bobbins 40 should be replaced.
Therefore, work delay (stop) according to replacement of the bobbin 40 and remaining lower threads in the remaining bobbins 40 are entirely disposed and thus a resource is largely wasted.
If more lower threads can be wound in the bobbin 40, such a problem can be considerably solved, but an amount of wound lower threads is limited by a size of an external form of the bobbin 40. That is, in order to increase a load amount of the lower thread, a diameter or a width of the bobbin 40 should be enlarged.
However, when enlarging a diameter or a width of the bobbin 40, the following problems occur.
When a diameter or a width of the bobbin 40 is enlarged, a size of the BC for housing the bobbin 40 should be also enlarged, and when a size of the BC is enlarged, a supply length of the upper thread 3 that moves upward the lower thread 4 by hooking while revolving the BC should be extended.
However, a supply length of the upper thread 3 depends on a stroke distance of a thread take-up crank, and thus a size or a structure of the thread take-up crank should be also changed.
That is, there is a problem that a sewing machine should be newly designed and produced, and when a supply length of an upper thread increases, before the upper thread forms a stitch together with a lower thread, a length that performs a reciprocating motion by vertically penetrating a cloth increases and thus there is a problem that the upper thread itself is damaged and a stitch is not appropriately formed due to a frictional heat.
In order to solve such a frictional heat, it is necessary to reduce the reciprocating motion number of a needle bar, and when the reciprocating motion number is reduced, work efficiency is deteriorated and thus there is a problem that an increase effect of a load amount of the lower thread is decreased.
Therefore, a new concept of lower thread supply device that can increase only a load amount of the lower thread without greatly changing an existing sewing machine structure while maintaining the reciprocating motion number of the needle bar and a supply length of the upper thread 3 is requested.
Further, in a conventional lower thread supply device, as shown in FIG. 1, the hook body 20 is connected to an end portion of the driving shaft 10 and rotates in a high speed and thus even if the hook body 20 is unbalanced a little, there is a problem that the hook body 20 may greatly vibrate and particularly, when a diameter of the hook body 20 is extended, a possibility that such a phenomenon may occur is very high.